Earthquakes are some of the most terrifying of natural disasters.  They have been happening since the beginning of time, yet there are still many mysteries about them.  Some are large enough to destroy entire cities, while others are so small that humans can not even feel them.  There are many interesting things about earthquakes, so read on!        Even in ancient times, many people wondered about earthquakes.  One of the first machines made to give information about earthquakes was created in ancient China by a scientist named Chang Heng way back in A.D. 132.  Though the original machine is lost, there has been a replica made by descriptions of that time.  It consisted of a large 6 foot diameter bronze vessel with a pendulum hanging in the middle.  The pendulum was attached to five dragons mouths.  It was made so that when an earthquake too small to be felt occurred, the pendulum would swing, and, pulling the lever attached to the dragon opposite the side that the earthquake was on, the dragons mouth would open and drop a ball into a pot on that side.  After the discovery of electricity, many more complex earthquake detectors, called seismographs, were created.               The main way  to measure an earthquake is to use the Richter Scale.  The Richter Scale rates the earthquake's magnitude.  The measure of magnitude was created by a man named Charles Richter.   He created it to compare local earthquakes where he lived.  The ratings were made by looking at the reading off of a seismograph, a zero being the smallest.  Now, however, since the seismographs are much more sensitive, some earthquakes are given a negative rating.  The highest Richter magnitudes recorded are about a nine.        Overall, there are three different types of waves in an earthquake: the primary, or P, waves, the secondary, or S waves, and the surface waves.  The first waves to arrive are the P waves.  They move quickly, and often aren't felt.  The secondary waves come second.  They move slower than the P waves, and make small, jagged cracks appear in the ground.  The slowest waves of all are the surface waves.  They move in the most complicated way, and often cause the most damage in all of the earthquake.        Unfortunately, earthquakes are often the cause of many more natural disasters.  They can start avalanches up high in the mountains, and tsunamis when they are underwater.  They can also be the source of huge landslides that destroy wildlife and buildings.  Due to better technology, though, we are getting to where we are able to give better safety warnings.  In the future, we may even be able to prevent people getting injured during earthquakes.  Until then, though, we will have to just play it safe!